[en] Mind-wandering refers to the occurrence of thoughts whose content is both decoupled from stimuli present in the immediate environment and unrelated to the task currently being carried out. In a series of experiments, we used a newly designed experience sampling method to assess mind-wandering episodes and to distinguish them from other kinds of distractions (irrelevant interoceptive/exteroceptive sensory perceptions and interfering thoughts related to the appraisal of the current task). In Experiment 1, we examined the impact of mind-wandering on performance of the Sustained Attention to Response Task (SART; a Go/No-Go task). Analyses demonstrated that episodes of mind-wandering impair SART performance to the same extent as irrelevant sensory perceptions. In Experiment 2, we focused on the content of mind-wandering in order to assess its possible functions. We observed that most of reported mind-wandering episodes refer to the anticipation and planning of future events. Furthermore, this “prospective bias” was increased when participants’ attention had been oriented toward their personal goals prior to performing the SART. In Experiment 3, we examined the neural correlates of mind-wandering using functional magnetic resonance imaging. The results showed that the brain regions that were more active during episodes of mind-wandering are similar to the regions that have been associated with imagining future events in previous studies. Together, these results suggest that although episodes of mind-wandering negatively impact current task performance, they may have important adaptive value and could, in particular, play a key role in planning for the future.